Re: [RFC PATCH 1/3] mm/memcg: Protect per-CPU counter by disabling preemption on PREEMPT_RT

Waiman Long
2021-12-23T16:01:37Z

On 12/23/21 02:34, Sebastian Andrzej Siewior wrote:
> On 2021-12-22 21:31:36 [-0500], Waiman Long wrote:
>> On 12/22/21 06:41, Sebastian Andrzej Siewior wrote:
>>> The per-CPU counter are modified with the non-atomic modifier. The
>>> consistency is ensure by disabling interrupts for the update.
>>> This breaks on PREEMPT_RT because some sections additionally
>>> acquire a spinlock_t lock (which becomes sleeping and must not be
>>> acquired with disabled interrupts). Another problem is that
>>> mem_cgroup_swapout() expects to be invoked with disabled interrupts
>>> because the caller has to acquire a spinlock_t which is acquired with
>>> disabled interrupts. Since spinlock_t never disables interrupts on
>>> PREEMPT_RT the interrupts are never disabled at this point.
>>>
>>> The code is never called from in_irq() context on PREEMPT_RT therefore
>> How do you guarantee that these percpu update functions won't be called in
>> in_irq() context for PREEMPT_RT? Do you think we should add a
>> WARN_ON_ONCE(in_irq()) just to be sure?
> There are no invocations to the memory allocator (neither malloc() nor
> free()) on RT and the memory allocator itself (SLUB and the
> page-allocator so both) has sleeping locks. That means invocations
> in_atomic() are bad. All interrupt handler are force-threaded. Those
> which are not (like timer, per-CPU interrupts or those which explicitly
> asked not to be force threaded) are limited in their doing as they can't
> invoke anything that has a sleeping lock. Lockdep or
> CONFIG_DEBUG_ATOMIC_SLEEP will yell here.
> The other counter are protected the same way, see
> c68ed7945701a ("mm/vmstat: protect per cpu variables with preempt disable on RT")

Thanks for the explanation as I am less familiar with other PREEMPT_RT
specific changes.

Cheers,
Longman